With the rapid advance and continual progress in technology, the organic light emitting diode (OLED) technology has been provided and widely used in various applications such as TV, computer monitor, notebook computer, mobile phone or PDA. In general, the OLED display includes many OLED pixel circuits arranged in the form of a matrix, and each OLED pixel circuit includes an OLED element and a corresponding driving circuit. However, pixels of the conventional OLED device are controlled by thin-film transistors (TFT). Consequently, the pixels of the conventional OLED device inherit the disadvantages of the TFTs and would be aged along with using time.